Is there a way to override a material when working with UVs?
I believe it's not currently possible (as of December 2018) with a simple UI toggle. A UV checker overlay that doesn't affect the materials is one of the most loved features proposed by the community, but still not implemented.



mockup by Xuri Greer

I think that we will see this feature in the (very) near future, since it's known that the devs have discussed it already.



A workflow I normally use is:


Add a Shader editor to the "UV Editing" workspace, and switch the Viewport to LookDev


Make your actual material single user and create a different "Diagnostic material" with a textured Emission shader (add a UVMap input if it's not unique)


When you change the texture in the editor, update the material too




After having this set up, it's just a matter of one more click each time you switch.

(You may prefer using the "Node Wrangler addon"'s preview feature instead of a whole new material)